This invention relates to the steering of a drill bit at the end of a drill string within a borehole to permit drilling of the borehole along a deviated path.
It is known to drill a section of borehole along a deviated path with respect to the vertical by use of a downhole trajectory control device which causes the drill bit located at the end of the drill string to be tilted to permit drilling at an inclined angle with respect to the immediately preceding section of the borehole. Such a trajectory control device may be constituted by a bent sub or a bent motor housing which may be installed in the bottomhole assembly close to the drill bit so as to angularly offset the drill bit so that, when the drill string is maintained at a required angular orientation, rotation of the drill bit by a downhole motor results in drilling along a curved path.
Conventionally, when drilling is to be initiated along a deviated path, the drill string is withdrawn from the borehole to enable a bottomhole assembly incorporating a bent sub or bent motor housing to be fitted to the end of the drill string before the drill string is again introduced into the borehole. However such tripping of the drill string may result in a substantial loss of drilling time.
It is also known to make use of a trajectory control device, such as a variable angle bent sub, which is adapted to permit the curvature of the section of deviated borehole being drilled, that is the so-called "build rate", to be varied without requiring the drill string to be withdrawn from the borehole to change the bottomhole assembly. Such a variable angle bent sub is disclosed in GB 1494273 and is controllable from the surface to vary the angle of the bend in the sub.
It is also known to make use of a trajectory control device in the form of a variable stabilizer, such as is disclosed in U.S. Pat. No. 4821817, in which equiangularly distributed stabilizer blades, which normally engage the borehole wall in order to centre the drill string within the borehole, are retractable in order to permit the drilling angle of the drill bit to be changed. Control of such a trajectory control device may be effected by operation of the mud pump at the surface to change the flow rate of the drilling mud which is pumped down the borehole to lubricate the drill bit and bring the drilling cuttings to the surface.
However these various trajectory control devices suffer from a number of disadvantages in use, either because they are difficult to control and do not operate satisfactorily in the field, or because they suffer from inherent limitations in use. Thus, for example, the use of a bent motor housing suffers from the disadvantage that, in order to drill a relatively straight section of borehole with a bottomhole assembly including such a bent motor housing, it is necessary to simultaneously rotate the drill string and actuate the downhole motor, and this results in eccentric rotation of the drill bit producing a section of borehole (of, it is believed, helical shape) of a greater internal diameter than is usual, thus increasing bit wear, producing higher static and dynamic stresses in the bottomhole assembly and increasing drill string rotating torque and axial drag.
In this specification terms such as "upper", "lower" and "bottom" are used for convenience to denote parts which have such an orientation in the drill string when the drill string extends substantially vertically downwardly in a borehole. However it will be understood that these parts may have a different orientation when the bottomhole assembly is in a section of borehole which deviates substantially from the vertical, and which may even be substantially horizontal.
It is an object of the invention to provide novel means for steering the drill bit which overcomes many of the disadvantages of known trajectory control devices.